In an automobile coating line, since works of different coating colors are conveyed together, rotary atomizing electrostatic multi-color coating machines of supplying coating materials of respective colors selectively to a coating machine and conducting color-change coating with an optional coating color have been used.
FIG. 7 shows such an existent electrostatic coating machine 31 having a rotary atomizing head 33 driven rotationally by a built-in air motor 32.
In the rotary atomizing head 33, an inner bell 36 is attached to an outer bell 35 attached to the top end of a tubular rotary shaft 34 of the air motor 32, and a coating material chamber 37 is formed between the rear face of the inner bell 36 and the outer bell 35.
Then, a coating material of a color selected by a color-change device (not illustrated) is supplied through a fine tubular nozzle 38 inserted in the tubular rotary shaft 34 to the coating material chamber 37, flows out from a coating material discharge hole 39 penetrated in the peripheral surface of the coating material chamber 37 by a centrifugal force along the inner surface of a rim portion 40 of the outer bell 35 and atomized under rotation at an atomizing edge 41 formed at the top end thereof.    Patent Document 1: JP-A No. 9-94489    Patent Document 2: JP-A No. 2003-374909
According to this, when a coating material of a coating color for a preceding work is supplied from the fine tubular nozzle 38 while rotationally driving the coating material rotary atomizing head 33 by the air motor 32, the coating material flows into the coating material chamber 37 , hits against the rear face of the inner bell 36, is blown to the peripheral surface of the coating material chamber 37 centrifugally by the rotation thereof, flows out from the coating material discharge hole 39 to the rim portion 40 and atomized at the top end thereof.
Then, in a case where the coating color of a succeeding work is different, a cleaning fluid such as a thinner (cleaning fluid) and air is supplied from the fine tubular nozzle 38 to the rotary atomizing head 33 before reaching of the work to clean the coating material of the preceding color remaining in the coating machine 31 and then a coating material of a succeeding color is supplied.
By the way, reduction of VOC (volatile Organic Compounds) and CO2 has been demanded recently in view of environments and, in a case of conducting color-change coating, color-change cleaning has to be conducted within a restricted period of time on every completion of the coating for the preceding work till reaching of the succeeding work and since color mixing is caused to result in coating failure in a case where the cleaning is insufficient, the amount of thinner used for cleaning can not be decreased extremely.
Particularly, since the thinner supplied from the fine tubular nozzle is jetted directly, the rear face of the inner bell 36 is cleaned easily. However, since the ceiling of the coating material 37 can not be cleaned unless the coating material chamber is filled with the thinner, the amount of use thereof can not be decreased.
Further, while the coating material supplied to the coating material chamber 37 flows out from the coating material discharge hole 39 penetrated in the peripheral surface thereof along the inner surface of the rim portion 40 of the outer bell 35 by a centrifugal force and atomized under rotation by the atomizing the 41 formed to the top end thereof, the coating material is not always supplied uniformly to each of the coating material discharge holes 39 formed in the peripheral direction when the centrifugal force exerts on the coating material in the coating material chamber 37.
Accordingly, the coating material is not discharged uniformly over 360° with the rotary atomizing head 33 as a center. While it is supplied in a greater or a smaller amount depending on the sites. Since the sites change at random with lapse of time and they are under a substantially uniform coating layer is formed entirely.
However, according to the experiment made by the inventor, it has been found that sites supplied with a larger amount and a smaller amount interfere to each other as a result of random change of them to sometimes result in sites where the coating layer is thick or thin although at a slight possibility.